1. Technical Field
The present invention relates to a protection circuit that detects over-charge and over-discharge of a storage battery and protects the storage battery, and relates more particularly to technology that reduces the power consumption required to detect over-charge and over-discharge states.
2. Related Art
Lithium ion batteries are one type of rechargeable storage battery. Lithium ion batteries offer high voltage, high energy density, and no memory effect or loss of capacity due to repeated shallow discharge/charge cycles, and are therefore widely used in portable electronic devices such as cell phones, notebook computers, and portable computing devices such as personal digital assistants (PDA).
Storage batteries generally require a protection circuit that monitors charging and discharging in order to maintain quality and safety. In addition to the normal operating voltage range being extremely close to the voltage range of the over-charge and over-discharge danger zones, however, the safety and quality of lithium ion batteries in particular are greatly affected by over-charging and over-discharging. As a result, protection circuits used with lithium ion batteries must consistently provide very precise detection of over-charge and over-discharge states, typically on the level of several 10 mV. Lithium ion batteries are protected by turning charging off when an over-charge state is detected, and turning discharging off when an over-discharge state is detected.
Reducing power consumption in order to extend drive time (operating time) is desirable in portable devices that use lithium ion batteries and other types of storage batteries. Reducing power consumption in the protection circuit that monitors charging and discharging is therefore also desirable. In order to reduce power consumption in an over-charge detection circuit and over-discharge detection circuit that detect over-charge and over-discharge states by comparing the storage battery voltage with a reference voltage with high precision, Japanese Unexamined Patent Appl. Pub. JP-A-H08-23639 teaches driving the over-charge detection circuit and over-discharge detection circuit in cycles with a specific gap therebetween instead of constantly.
Storage batteries are also now commonly used in small devices such as wristwatches. In addition to reducing the size of the storage battery, this means that battery capacity is also reduced. Even greater efficiency is therefore required in the protection circuit in order to further extend the drive time and improve product utility in devices that use a storage battery with relatively small battery capacity.
Power consumption could be reduced by further increasing the interval between the operating periods of the over-charge detection circuit and over-discharge detection circuit as taught in JP-A-H08-23639, but this means that detecting the danger zone may be delayed, impairing the ability to maintain quality and assure safety.